[Mesa-dev] [PATCH 0/7] RadeonSI 32-bit pointers v2 & Gallium changes

[Marek Olšák]

On Fri, Feb 2, 2018 at 11:39 PM, Marek Olšák <maraeo at gmail.com> wrote:
> On Fri, Feb 2, 2018 at 10:26 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>> Am 02.02.2018 um 21:48 schrieb Marek Olšák:
>>> Hi,
>>>
>>> This is the second and hopefully final version of 32-bit pointer
>>> support for radeonsi.
>>>
>>> Constant buffer 0 now has restrictions on which buffers can be set
>>> in that slot.
>>>
>>> I plan to push this when my LLVM patch lands in 6.0 (hopefully it
>>> will be accepted there).
>>>
>>> There will also be a dependency on new libdrm (not included in this
>>> series).
>>>
>>> Please review.
>>>
>>
>> From a api cleanliness point of view, I don't like this much.
>> First, you're making the hack case the default and even require it. IMHO
>> a driver should be able to bind ordinary UBOs to all buffer slots. This
>> is really not a nice burden to put on state trackers to do something
>> special for just slot 0. The gallium API should stay reasonable imho,
>> that's a bit too much custom tailoring for GL for my liking.
>>
>> Maybe I'm missing something but I can't quite see why you can't handle
>> this transparently inside the driver. Can't you just create a different
>> shader depending on what kind of buffer is bound or what's the problem?
>> (You wouldn't expect it to change therefore you should not have to
>> recompile.)
>
> We don't recompile shaders in the vast majority of cases. When shader
> compilation stalls rendering, the gaming experience is destroyed.
>
> There is no alternative. Our shader ABI will be set up such that it
> only has 32-bit pointers in shader registers. There are
> performance-related reasons for that.
>
> We'll handle maintenance and testing of this feature. You won't have
> to do anything.

The CAP is only a formality. In reality, we only have to fix
vl_compositor.c to use pipe_context::const_uploader::flags (or the
CAP) and that's it. All other code we care about is unaffected (GL,
VDPAU, VAAPI, OpenMax, Nine).

There are no users binding a real buffer as constant buffer 0 other
than VL and XA.

Marek